Recent models of passenger car tires are in many cases provided with a rib pattern, or a tread pattern that has straight grooves extending circumferentially around the tread, because it offers the following advantages: small rolling resistance, good ride characteristics, good steering stability, low noise that develops between the tire and the road surface, and less likelihood of "hydroplaning" in the presence of a water film on the road surface.
In order to further reduce the noise that develops between the tire and the road surface and to suppress more effectively the occurrence of "hydroplaning" in the presence of a water film on the road surface while enhancing the drive and brake forces, various approaches are being adopted. These approaches include increasing not only the number of straight grooves, but also the number of lug grooves that intersect with the straight grooves to extend transversely across the tread, as well as increasing the angle the lug grooves form with respect to the straight grooves.
However, if the number of straight grooves and lug grooves are simply increased in the conventional pneumatic radial tires, the area of each of the blocks that are formed by the straight grooves and lug grooves will decrease, leading to lower rigidity of the blocks and poor steering ability.